Ok, two minutes work swapping to the '92 card. Specifically this one
was enough to solve for me.
Card Type: AirPort Extreme (0x168C, 0x8F)
Firmware Version: Atheros 9280: 220.127.116.11-P2P
Country Code: AT
Supported Channels: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140
Wake On Wireless: Supported
I do have to say though that I re-installed the IO80211Family.kext as before I did that even the new card was not doing the business for me. It worked fine, but did not support airdrop to begin with, I suspect something I did to the kext previously when trying to get the '72 card working stopped the '92 card from getting airdrop. So I took the kext from my new MacBook Air and installed that.....after that it was bingo!
I will out of curiousity go back and check again now the '72 card to make sure there was nothing magical about the MBA kext!
EDIT: No, no magic. Swapping back to the '72 - airdrop disappears, change back again to the '92 and back it comes.....my advice: go for the '92!!